Our recent work on the structure of Limulus thick filaments indicates that the substructure of invertebrate thick filaments may be amenable to analysis by a combination of high resolution electron microscopy, optical diffraction analysis, and computer reconstruction techniques. I propose to extend this approach to the study of thick filaments from several additional invertebrate muscles which exhibit pronounced differences in paramyosin to myosin molecular ratios and/or physiological properties. The overall objective of the proposed research is to elucidate the packing arrangement of the proteins myosin and paramyosin in thick filaments of these different muscles and to relate differences in thick filament structure to the physiological properties of these tissues. Specifically, we will determine: 1) the helical arrangements; and 2) rotational symmetries of the filaments myosin cortices; and 3) the sizes and structures of their paramyosin cores, in lobster fast and slow abdominal muscles, scallop striated abductors, giant waterbug air-tube retractors and giant barnacle scutal depressors. These parameters will be correlated with the paramyosin to myosin ratios, biochemical similarities and differences, and the functions of the individual muscles. The results of these studies will provide insight into the structural bases for functional differences among contractile tissues in general, and may help to clarify thick filament structure in different types of vertebrate muscle (striated, smooth) as well.